Fiberoptic cables, having diameters measuring less than 0.00015 inch, can transmit multiple signals containing considerable quantities of information for hundreds of miles. The ability to carry multiple signals derives from the ability of the fiberoptic cable to "multiplex," or simultaneously transmit different light signals, each having a different wavelength. Multiplexed fiberoptic communication requires that the wavelength of the light sources introduced into the receiving end of the cable be adjustable to any wave length in the 1300 nm to 1600 nm range.
A typical laser light source includes a gain medium, or a semiconductor optical amplifier (SOA). One side of the SOA has an antireflection (AR) coating. The other side of the SOA is uncoated or has a high reflection coating. Light emitted from the AR-coated side is trained by one or more lenses onto a thin film filter, typically mounted on a substrate. The filter passes light in a range of wavelengths, thereby enabling a narrow linewidth or single mode laser emission. The filtered light is trained by another lens onto a curved mirror, or a lens and a flat mirror, which reflects the light back into the filter and the SOA. To ensure that out-of-band light does not return into the SOA, with undesirable consequences, the filter is positioned such that the angle of incidence with respect to the projection line of the light is not 90.degree., or orthogonal to the projection line. Out-of-band light is not passed through the filter, but reflected away from the SOA The wavelength of the laser emission is determined by the overlap between the transmission wavelength of the filter and the modes of the laser cavity.
In general, a problem encountered with typical film filters is the existence of temperature drifts, or gradients, in the film. Temperature drifts cause undesirable wavelength drifts and associated mode hopping and noise. An ideal laser light source for multiplexed communication must provide light with a stable wavelength.
To provide light at variable wavelengths, some light sources include a plurality of lasers, each emitting light at a different wavelength. However, normal wear and tear or the unavailability of a lasers at specific wavelengths can limit multiplexing potential.
Other light sources employ lasers with an angularly-adjustable filter. In such cases, rotating the filter changes the angle of incidence between the filter and a predetermined projection line, which in turn changes the transmission wavelength of the filter.
A major disadvantage of rotating angle-tunable lasers is that tuning the transmission of wavelength of the thin film filter necessarily is accompanied by an increase in optical path length in the underlying substrate. This can cause undesirable wavelength and intensity instabilities absent a high degree of controlling and stabilizing the rotation angle.
Still other light sources alter emission wavelength with a filter that has a variable Fabry-Perot gap thickness along its length. In these situations, translating the filter along a predetermined plane positions a portion of the filter having a different thickness in line with a predetermined projection line. The thickness difference correspondingly alters transmission wavelength.
A major disadvantage of the filter-translating tuning approach is that, once the filter is located, it must be maintained so that it does not drift into transitions zones between portions of the filter having continuous thickness, causing wavelength drift. The effect of filter drift on wavelength variation can be minimized by increasing the projected spot size of the laser beam at the filter. However, the filter must be long enough to cover the desired wavelength range. Furthermore, the cost of fabricating such filters, with large wavelength variation over a few millimeters distance, as is desired for compactness, is high.
The foregoing demonstrates a need for a singular, compact, tunable light source that emits light with variable, but stable, wavelengths and stable intensity that is thermally and mechanically insensitive.